You can easily rate the clips you like and the clips you don’t like as you review them. When you’re ready to make a movie or trailer, you can focus on your best footage by displaying only those clips you marked as favorites.
In the iMovie app 
on your Mac, select a range, a clip, or multiple clips in the browser.
Do one of the following:
If you like the selection: Choose Mark > Favorite (or press F).
In the browser, a green line appears at the top of ranges you’ve marked as favorites.
You can also hold down the F key while a clip is playing to mark a favorite range. When the playhead reaches the end of the section you want to mark as favorite, release the F key.
If you don’t like the selection: Choose Mark > Reject (or press the Delete key).
A red line appears at the top of frames you’ve marked as rejected.
Note: If you chose Hide Rejected from the pop-up menu to the left of the browser search field, clips marked as rejected disappear from view.
In the iMovie app on your Mac, make sure the clips you want to work with appear in the browser.
You might need to click the pop-up menu to the left of the browser search field and choose All Clips so that all the clips you need to access are visible.
In the browser, select the clips whose ratings you want to remove, and choose Mark > Unrate (or press U).
The green or red line at the top of the clips disappears.